Once saved always saved is defined just as it sounds, that is, once a person becomes a Christian he will forever remain saved. Sometimes the same doctrine is called eternal security or even the perseverance of the saints by the Calvinists, but they mean the same. The omitted part of the general description of once saved always saved is, the proponents also think one will remain saved regardless how they sin, become doctrinal apostates, fruitless, lukewarm, unforgiving, hateful, greedy, etc.! Hence, they believe there are two types of adulterers, two types of murderers, two types of thieves, two types of liars, two types of witches, two types of swindlers, etc. – one type is saved because of a past point of faith and the other type of adulterer, murderer, thief, liar, witch and swindler isn’t saved because he never, at any pointed in his entire life, believed on Jesus! To them, one’s present behavior is irrelevant in revealing who is a Christian and who isn’t. In fact, they don’t believe you can say another person is unsaved, even if he is a notorious serial rapist and murderer, since he could have had faith in Jesus at an earlier time. The BTK serial killer is an example of that. That is critical information, which should be readily understood about that doctrine, but often is concealed. When it is understood, it becomes apparent that it is a license to sin and the same kind of counterfeit grace message, which Jude 3,4 states ungodly people teach.
[Dan Corner has authored the most exhaustive and comprehensive book ever written refuting eternal security. That 801 page book (hardback or softback) is “The Believer’s Conditional Security,” and is readily available through Evangelical Books.]